Just an update for anyone interested in this saga (and there seem to be quite a few), especially those who have provided such useful information following my query:-
I now have Zedekiah Oakley's Will of 1865, which is mercifully brief. But vitally it names his son Thomas and four daughters as beneficiaries, all of whose names tally, and one of whom is Elizabeth Rebecca Lebon, nee Oakley. I also have her marriage record to John Lebon in 1843, in which her father is named as HEZEKIAH Oakley - so this handily confirms that Hezekiah and Zedekiah are the same person - unless I'm missing something!
It doesn't explain why he decided to change back to his birth name late in life, but at least we can be fairly confident its the same family, back to the Zedekiah b. in Glemsford in 1794, and his probable parents Samuel and Mary. It also doesn't explain how a Brewer's Servant/Labourer managed to leave an estate of "value under £450" in 1865 according to Probate, ie maybe about £30,000 in today's money I believe...
For those interested, the Sewell / Arnall tree on Ancestry gives a lot of information on this family, which tallies well with my own findings so far, so I'm inclined to give it a good deal of cautious credence.
I anyone has anything to add, I'd be very interested to hear from them !
Potterer.